During early development in a model embryonic system three proteins have been found to somehow stimulate the expression of specific gene products. The three proteins are in the extracellular matrix of the sea urchin embryo and each stimulates the synthesis of specific marker genes at known times in development. The marker molecules are different in each case and are germ layer-specific. The overall goal of the project is to learn how each extracellular matrix protein might stimulate such specific responses. The three specific aims indicate the observations and the goals of each protein. I. Hyalin. At fertilization an extraembryonic membrane is assembled surrounding the zygote. The major protein forming that matrix is hyalin. When cells release from hyalin, an actin gene is up-regulated. Cells can be released from hyalin experimentally with the addition of a monoclonal antibody to hyalin. Associated with that release is an up regulation of actin. Studies in this section are to determine the sequence of events in the experimental system starting with hyalin and ending with increased expression of an actin gene. These studies will include characterization of the hyalin fragment recognized by cells, identification of the hyalin receptor, and determination of how the membrane signal might be transduced intracellularly. II. Echinonectin. In a search for the sea urchin equivalent of fibronectin this protein was identified. It is a better adhesive substrate in the sea urchin embryo than is fibronectin. The goal is to determine whether echinonectin is related to fibronectin, and to determine the means by which this molecule is necessary for the expression of a primary mesenchyme marker identified by the monoclonal antibody lg8. The experiments will characterize echinonectin which has been purified to homogeneity, and for which there is a monospecific affinity purified polyclonal antibody. III. Collagen. If one prevents collagen from being deposited into the extracellular matrix the embryo reaches the mesenchyme blastula stage normally but gastrulation is inhibited. If the collagen block is removed the arrested embryos resume development and gastrulate. The goal is to determine how collagen deposition into the extracellular matrix is a critical for the expression of endoderm-specific gene markers.